The present invention relates generally to gas turbine engines, and, more specifically, to compressor rotors having curvic couplings.
One type of gas turbine engine includes an axial compressor having several stages joined together using a central tie rod. The compressor is typically formed in axial segments, with one or more rows of compressor blades or airfoils extending radially outwardly from a common annular rim. Adjacent rims use curvic coupling teeth at opposing axial ends thereof which engage each other to circumferentially interlock the rotor segments when axially clamped together using the tie rod.
Curvic couplings are commonly used in axial compressors of small gas turbine engines. The coupling is defined by the complementary curvic coupling teeth which extend axially outwardly from respective rotor rims and are circumferentially spaced apart from each other. The complementary curvic teeth of the adjacent rims engage each other to interlock the adjacent rims and carry torsional loads therethrough.
Each rotor segment is typically dynamically balanced prior to assembly with the adjacent rotor segments. Balancing is conventionally effected by providing a specifically sized balance ring at each end of the rotor adjacent to the curvic teeth. The balance ring is initially sized in axial and radial extent so that a full axial notch extending radially outwardly into the balance ring may be machined for removing a specified amount of material for effecting dynamic balancing of the rotor in the two planes defined at opposite axial ends of the rotor.
Since the balance notch splits the balance ring at one circumferential location, the ring no longer extends a full 360.degree. in circumference and therefore loses its hoop strength capability for carrying hoop loads. The remaining split ring, therefore, merely provides dead weight once the balance notch is created. The centrifugal force generated from the dead weight is carried during operation of the rotor by the portion of the rim adjacent to the curvic teeth.
The substantial centrifugal force generated by the rotating airfoils is carried by the portions of the rim directly below the airfoils and by an integral web and hub defining a disk specifically configured and sized for carrying centrifugal loads. The centrifugal loads are directly proportional to radius from the axial centerline of the rotor and rotational speed squared and therefore it is desirable to minimize mass at large radius, while at the same time providing a suitable disk for carrying the centrifugal loads. The disk hub has a relatively large cross sectional area for carrying the centrifugal loads which are transmitted in part from the airfoils and rim through the relatively axially narrow web in a conventional hour glass axial profile.
The outer surface of the rotor rim typically defines the radially inner flowpath surface of the compressor as air is compressed in turn from stage to stage. The aerodynamic requirements of the compressor determine the required shapes of the airfoils and their sizes including the radial position of the rotor rim. In one conventional axial compressor design, the rotor rim at the curvic teeth defines the life limiting location of the rotor due to conventional low cycle fatigue (LCF). The small rings used for balancing the rotor actually reduce the low cycle fatigue strength of the rotor at the curvic teeth since the balance notch splits the ring and eliminates its hoop strength, with the remaining split ring providing dead weight which must instead be carried by the rotor rim.
In some conventional designs, the LCF strengths may be increased by relocating the curvic coupling and corresponding teeth at a smaller radius provided by an integral support cone extending from the rotor rim. In this way, the curvic coupling is located at a smaller radius for a given design to decrease the circumferential loads for increasing LCF life. However, an additional annular flowpath spacer is then required between the adjacent stages to fill in the void created near the rotor rims by relocating the curvic coupling radially inwardly. This increases the cost and complexity of the design for achieving suitable LCF life.